A third-generation cellular wireless communication system using CDMA (Code Division Multiple Access) allows multimedia information communications via an IP network, and there is a growing need for more broadband and higher-speed communication services. It is expected that supports for broadband communications become common, such as, best-effort data communications, VoIP voice communications, and distribution of video and other streaming information. With such diversification of the usage scenes, there is a growing need for more broadband and higher speed services. In terms of broadband and high speed, an OFDMA wireless communication system attracts attention, as a next-generation wireless communication system after CDMA.
The OFDMA is a technology for improving the frequency usage efficiency by orthogonally multiplexing a plurality of orthogonal carrier waves in the frequency domain. Cellular wireless communication systems using OFDMA are positioned as beyond 3rd-generation cellular wireless communication systems, and representative specifications thereof are LTE (Long Term Evolution) and UMB (Ultra Mobile Broadband). These are globally standardized by business organizations such as 3GPP (3rd Generation Partnership Project) and 3GPP2.
In wireless communication systems, it is known that, to wireless access that connects a terminal and a base station, wireless wave interference from an adjacent base station (cell) and a terminal belonging, to the adjacent base station is fatal. In particular, in OFDMA wireless communication systems, it is known that a large influence is caused by interference from an adjacent base station (cell), and thus it is very difficult to design an arrangement of base stations.
In wireless access from a cell edge that is the border between areas covered by base stations, the interference power level from an adjacent cell is close to the desired signal level from the own cell, and this substantially causes a deterioration in SINR (signal to interference and noise power ratio), which is an index indicating the channel quality.
Reducing interference between adjacent cells is very important to improve the channel capacity for wireless access in the whole system. To reduce interference between adjacent cells, the most important thing is not to transmit excess power. However, when the power is strongly suppressed, there arise problems such as a reduction in cover area (cell diameter) and a reduction in transmission speed. Therefore, it is necessary to specify excess power and to reduce the power.
As a countermeasure against interference between adjacent cells, there is a technology called FFR (fractional frequency reuse), in which frequency bands at which transmission can be performed at high power are individually allocated to the cells and are used to cover terminals located at the cell edges (the edges of the cells). In mobile WiMAX, a channel format is defined in which the frequency is divided into three and used (IEEE Standard for Local and metropolitan area networks Part 16: Air Interface for Broadband Wireless Access Systems (IEEE Std 802.16 TM-2009), 29 May 2009). In FFR, processing of allocating high-power radio resources is performed by a scheduler provided in a base station. Therefore, the scheduler needs to be operated by always taking account of the cell edge and the cell center. FFR is effective for both the downlink and uplink in wireless access.
In addition, in an uplink operation in wireless access, transmit power control for a terminal located at the cell edge is also closely related to the interference control. The terminal located at the cell edge requires high transmission power in order to overcome path loss with respect to the base station. To suppress this power, it is necessary to allocate more radio resources to reduce the encoding ratio of error-correcting code. By reducing the power, interference is reduced, but the frequency-resource use rate is increased, thus increasing the load in wireless access. JP-A-2009-118016 describes a technology for changing a frequency resource use method in wireless access according to the access load of the own cell.
In addition to the above-described consideration for a reduction in interference in wireless access, it is also important to control load balancing between cells at a location where terminal connections are concentrated. If the load exceeds the throughput of the backbone of a base station, this makes it difficult to establish a call connection. In view of wireless access and backbone throughput, a study has been conducted in order to balance the load levels between cells.
As shown in FIG. 2, base stations vary in cell size and in the number of accommodated terminals, depending on the installation site and the intended use. For example, in an urban area and the like, terminals are densely located, and thus it is difficult to accommodate those terminals by just using a base station having a large cell, such as a macrocell, in terms of the load of the base station. Therefore, smaller base stations having cells smaller than that of the macrocell 201-a, such as microcells and picocells, are dotted to support the macrocell. Hereinafter, the magnitude of transmission power of a base station is defined as the size of the cell. Further, a base station having a cell larger than another is defined as a macrocell, and a base station having a cell smaller than the macrocell is defined as a picocell, and these descriptions are used.
As described above, a picocell is installed particularly at a location where the terminal connection load is concentrated. Therefore, if load control is not appropriately performed, the load is heavily imposed on the picocell 201-b, and the load is lightly imposed on the macrocell 201-a. As shown in FIG. 3, there may be a large difference in the number of terminal connections between adjacent base stations.
In order to balance the terminal connection load, handover from the picocell to the macrocell is facilitated. A terminal that has been handed over from the picocell to the macrocell is located at a cell edge of the macrocell, which is the handover destination. The terminal requires higher transmission power to overcome the path loss with respect to the macrocell base station. In other words, when the terminal is handed over from the picocell to the macrocell for the purpose of the load balancing, this influences the picocell as large interference.